Boundary layer control on interstage guide vanes of a multistage centrifugal compressor in a refrigeration system



July 2, 1968 n. c. HOFFMAN 3390,55

BOUNDARY LAYER CONTROL ON INTERSTAGE GUIDE VANES OF A MULTISTAGE:CENTRIFUGAL COMPRESSOR IN A REFRIGERATION SYSTEM Filed June 28, 1967 2Shee'cs-Sheeil l INVENTOR.

DAVID C. HOFFMAN @uw Qa July 2, 1968 D, c. HOFFMAN 3,390,545

BOUNDARY LAYER CONTROL ON INTERSTAGE GUIDE VANES OF A MULTISTAGECENTRIFUGAL COMPRESSOR IN A REFRIGERATION SYSTEM Filed June 28, 1967 2Sheets-Sheet 2 INVENTOR. DAVID C. HOFFMAN nited States Patent C3,390,545 BOUNDARY LAYER CONTROL N INTERSTAGE GUIDE VANES 0F AMULTISTAGE CENTRIF- UGAL COMPRESSOR IN A REFRIGERATION SYSTEM David C.Hoffman, Stoddard, Wis., assignor to The Trane Company, La Crosse, Wis.,a corporation of Wisconsin Filed June 28, 1967, Ser. No. 649,583 8Claims. (Cl. 62-510) ABSTRACT 0F THE DISCLOSURE Separation and stall atthe interstage guide vanes of a multistage centrifugal compressor arereduced by using a relatively high pressure jet of fluid to energize theboundary layer adjacent each guide vane in the region where separationnormally occurs. Flash gas formed in an intermediate pressure economizerchamber located in the refrigeration system in which the centrifugalcompressor is installed is employed as the boundary layer energizingfluid.

Background of the invention In gas pumping machinery a flow conditionknown as separation almost invariably occurs. This is the phenomenonaccording to which fluid streamlines leave the solid bounding walls of apassage 0r body rather than following the bounding surface. Asstreamlines separate from a wall, they divert the boundary layer fromthe wall, causing sheets of vorticity to leave the wall. The fluid inthe separated region normally takes the form of a bubble. Separationreduces performance efficiency because of the pressure losses which itproduces in the main fluid stream. The most significant loss is theincreased shearing force produced as the main flow passes over aseparated region. A separated bubble will normally cause a shear loss inthe main flow stream many times that caused by a wall. Other flow lossesare generated as the separated fluid mixes with the main flow downstreamfrom the region of separation. The condition of extreme separationcausing the accumulation of large quantities of stagnant fluid, andoften unsteadiness adjacent the portion of the bounding surface fromwhich the flowing fluid has separated, is known as stall. The periodicshedding of the accumulated stagnant fluid causes the unsteadiness.

Separation and stall are limitations on the performance of any fluidhandling device. Most gas handling machinery is designed with the flowon the verge of, but not quite separated, for this is where the highestperformance is usually achieved. Separation in compressors is probablyresponsible for the onset of surge at a flow slightly below the peakefiiciency range. If the load point where stall commences can beincreased, then a concomitant increase in performance will invariably berealized.

Boundary layer energizing is one method by which stall and separationmay be eliminated or minimized. This involves supplying additionalenergy to the particles of fluid which are being retarded in theboundary layer. One way in which this may be accomplished is to inject asecondary stream of relatively high pressure fluid to increase themomentum of the boundary layer near the wall or surface over which thefluid is flowing. It is known that this can be accomplished bydischarging fluid from the interior of a moving impeller blade or wingover which the fluid is moving, or by injecting a portion of the highenergy fluid being pumped into the low energy separation zone thru slotscorrectly placed in the fluid handling device. See for example,Streeter, Handbook of Fluid Dynamics, McGraw-Hill, 1961, and U.S. PatentNos. 3,069,072 and 3,237,850.

Brief summary of the invention I have applied the basic concept ofboundary layer energization by injecting economizer gas in a unique wayto improve the fluid flow efliciency of the interstage, stationary guidevane portion of a multiple stage centrifugal compressor. The centrifugalcompressor of my invention is employed to compress refrigerant gas in arefrigeration system consisting of a compressor, condenser, economizer,and an evaporator interconnected in refrigerant flow relationship. Theeconomizer is a vessel or chamber interposed between the condenser andthe evaporator to effect a pressure reduction of the liquid refrigerantbefore it enters the evaporator. As the liquid is reduced in pressure inthe economizer a portion of it flashes to vapor, cooling the remainingliquid. The flash gas from the economizer is normally directed to anintermediate pressure stage of the refrigerant compressor. According tomy invention, an interstage wall of the compressor is provided with aplurality of angled nozzles each of which has its outlet disposedadjacent the suction side of one of a number of fixed guide vaneslocated in the crossover passage between stages of the compressor.Refrigerant gas from the economizer 4is directed through the nozzlesinto the crossover passage. The location and angle of the nozzles issuch that economizer gas is injected into the separated flow region nearthe trailing surface of each fixed guide vane. In this way, the boundarylayer adjacent each fixed guide vane is energized and the lossesnormally associated with separation and stall are greatly minimized ifnot eliminated.

Brief description of the drawings FIGURE 1 is a front elevation view,partly in section, showing the improved compressor of my invention.

FIGURE 2 is a vertical section view taken along line 2-2 of FIGURE 1,showing the fixed interstage guide vanes and the economizer injectionnozzles.

FIGURE 3 is a schematic view showing the manner in which my improvedcompressor is located in a refrigeration system utilizing an economizer.

FIGURE 4 illustrates the gas flow pattern between interstage guide vanesWithout boundary layer control.

FIGURE 5 illustrates the improved gas flow pattern between guide vanesachieved by using the economizer gas injection nozzles of thisinvention.

Description of the preferred embodiment With reference to FIGURES l and2 of the drawings, the multiple stage compressor 1 of my invention iscomprised of an outer casing 4 through which a drive shaft 2 extends.Shaft 2 may be driven by any convenient means such as a turbine orengine, not shown. Mounted upon drive shaft 2 for rotation therewith area first-stage impeller wheel 6 and a second stage impeller wheel 8. Acrossover passage generally indicated by reference numeral 13, having adischarge portion 14 and a return and diffusing portion 16, connects theoutlet of first stage mpeller 6 with the inlet of second stage impeller8. Suction gas is introduced into the inlet 12 of first stage impeller 6from a suction gas pipe 10. Gas compressed by first stage centrifugalimpeller 6 is discharged into passage 14 from which it is directed intodiffusing and return passage 16 towards suction inlet 18 of second stageimpeller 8. Blades 19 of impeller wheel 8 add energy to the gas toobtain the final discharge pressure. The `gas flows through dischargepassage 20 into chamber 21 from which it is `conducted out of thecompressor by discharge conduit 22.

Refrigerant gas ow into first stage impeller wheel 6 is controlled andthe capacity of impeller 6 is regulated by a first set of adjustableguide vanes 24. A similar set of adjustable guide vanes 26 are disposedupstream of the suction inlet 18 to second stage impeller 8. Linkagemechanism generally indicated by reference numeral actuates first stageguide vanes 24 and is driven by shaft 31 which extends outside of thecompressor casing 4 to a point of connection with control means notshown. A similar linkage mechanism 32 is employed to actuate secondstage guide vanes 26. Linkage mechanisms 30` and 32 are interconnectedfor synchronous operation by common drive shaft 28.

As is indicated in FIGURES 1 and 2, plate 36 which serves to defineinterstage return passageway 16 is provided with a plurality of fixedguide vanes 34 mounted thereon. The purpose of guide vanes 34 is to turnthe gas radially inwardly towards second stage suction inlet 18, and toassist in diffusing the gas delivered from the first stage ofcompression. The gas coming from the first stage of compression has arelatively high tangential component of velocity. This tangentialcomponent of velocity is substantially removed as the gas is guidedthrough passage 16 by vanes 34 and is turned radially inwardly. Theconversion of kinetic energy into static pressure is accomplished by theturning action of vanes 34 promoting diffusion of the total velocity ofthe compressed gas as it passes through the areas of graduallyincreasing cross section between adjacent vanes 34.

Because the refrigerant gas is being diffused inwardly in the directionof decreasing radius, blades 34 are highly loaded and are thereforequite susceptible to separation and stall. FIGURE 4 illustrates the fiowcondition which would normally develop between adjacent vanes 34. As isshown in that figure by reference numeral 15, a separated or stalledregion exists on the suction or low pressure side of guide vanes 34indicated by minus signs. Such a fiow condition has an adverse effect oncompressor etliciency as a result of the additional pressure losseswhich it creates in the gas stream flowing to the second stage suctioninlet 18. Furthermore, the zone of substantially no flow whichseparation produces adjacent `the trailing side of each vane 34 causes apoor flow distribution into second stage suction inlet 18.

In order to overcome these difiiculties and improve compressorefficiency as well as to provide a method of introducing economizer gas,I have provided a plurality of injection nozzles 40 in vane plate 36. Asis indicated in FIGURES 1 and 2, nozzles 40 are angled in order todirect economizer gas from chamber 38 generally parallel to theunseparated 'flow direction in return passage 16. Nozzles 40 arecircumferentially spaced about plate 36, each of said nozzles having itsoutlet disposed adjacent the trailing side of one of the guide vanes 34in the region where separation normally occurs.

The manner in which compressor 1 is connected in a standardrefrigeration system to receive a supply of economizer gas for injectionthrough nozzles is illustrated in FIGURE 3. Refrigerant gas dischargedby second stage impeller 8 passes through discharge conduit 22 tocondenser 42 where the refrigerant is condensed to a liquid. Fromcondenser 42 the refrigerant liquid passes through pipe 44 to economizervessel 46. In economizer vessel 46 the refrigerant liquid is lflashed toan intermediate pressure as the result of which the main body of liquidis further chilled and a certain amount of flash gas is formed. Thechilled refrigerant liquid is directed through conduit 52 to evaporator54 where it vaporizes to produce the desired cooling effect on asecondary liquid such as water flowing through coil 53. The vaporizedrefrigerant gas then passes through suction line 10 to the inlet offirst stage impeller 6. Flash gas formed in economizer 46 passes throughseparator 48 and then into pipe 50 which is connected to compressor 1.

With reference again to FIGURES 1 and 2, economizer gas flowing out ofpipe 50 is directed into plenum chamber 38 formed within compressorcasing 4 between vane plate 36 and second stage diffuser plate 25. Fromplenum chamber 38, the economizer gas is discharged through nozzles 40into the main stream of refrigerant flowing through return passage 16.The location and angular disposition of nozzles 40 are such thateconomizer Igas is injected tangentially with respect to curved trailingside 35 of each vane 34. The relatively high energy economizer gasserves to energize the stagnant or low energy boundary layer adjacenteach vane 34 and to prevent separation, thus establishing ow in a zonewhere there is normally substantially no flow. Nozzles 40 are tapered,and are sized and shaped so as to produce jets of economizer gas havingsubstantial velocity without creating such a back pressure as tosignificantly dampen the vaporization of refrigerant in economizer 46.

The improved flow pattern realized by economizer gas injection in theaforesaid manner is illustrated in FIGURE 5. The injection of highenergy economizer gas in the normally stalled region adjacent each vane34 serves to settle and reattach the flow of gas over the trailing side35 of vanes 34. As may be seen by the streamlines in FIGURE 5,separation is substantially eliminated and refrigerant gas follows thebounding surface of vanes 34 throughout their entire length. Gas is thusdischarged uniformly from the entire cross sectional area betweenadjacent vanes 34, and an even distribution of refrigerant gas all ofthe way around second stage suction inlet 18 is realized.

Thus the specific method of injecting economizer gas so as to energizethe `boundary layer on fixed guide vanes 34 provides several importantbenefits. First of all, separation and stall are substantially reducedif not e1iminated, thus greatly minimizing the flow losses normallyassociated therewith. Secondly, theimproved flow pattern producinguniform distribution of refrigerant gas into second stage suction inlet18 significantly improves the performance of second stage impeller 8.Thirdly, the noise generating tendencies of second stage impeller 8 arereduced by the uniform fiow of refrigerant gas therethrough.

Although I have shown and described a two stage centrifugal compressor,a compressor having any number of stages could obviously be employed,with economizer gas being injected at the appropriate intermediatepressure stage. I anticipate that various other modifications will occurto those skilled in the tart which will be within the spirit and scopeof my invention as defined by the following claims.

I claim:

1. In a refrigeration machine having a compressor, condenser,economizer, refrigerant Iliquid flow regulating means, and an evaporatorinterconnected in a closed circuit, said economizer comp-rising a flashchamber interposed between said condenser and said evaporator, and saidcompressor being of the multiple stage centrifugal type having axiallyspaced impe-ller wheels with crossover passage means therebetween, andguide vane means in said crossover passage means to turn the refrigerantgas owing from one of said impeller wheels and cause it to flow in aradial direction, the improvement comprising:

means for injecting refrigerant gas from said economizer into saidcrossover passage means near said guide vane means in such a way as toenergize the boundary layer adjacent the surface of said guide vanemeans, thereby minimizing flow separation from said guide vane means.

2. Apparatus as defined in claim 1 wherein:

said crossover passage means comprises a discharge passage portion and areturn passage portion leading inwardly towards the inlet of the nextone of said impeller Wheels;

and wherein said guide vane means comprises (a plurality of fixed,curved vanes circumferentially spaced within said return passageportion.

3. Apparatus as defined in claim 2 wherein:

said means for injecting refrigerant gas from said economizer in'to saidcrossover passage is so disposed and arranged as to direct said gas intothe normally stagnant boundary layer near the trailing side of each ofsaid guide vanes in a direction general-ly parallel to the main streamof refrigerant gas flowing `between said guide vanes.

4. Apparatus as defined in claim 2. wherein:

said compressor comprises an outer casing within which said impellerwheels are rotatably mounted;

and further including wall means within said casing and cooperatingtherewith to form an economizer gas plenum chamber, said chamber vbeingconnected to said economizer by a conduit extending through said casing;

and wherein said wall means includes a dividing wall separating saidreturn passage Iportion from said economizer gas plenum chamber;

and wherein said means for injecting refrigerant gas from saideconomizer into said crossover passage means comprises a plurality ofcircumferentially spaced openings in said dividing wall communicatingsaid economizer gas plenum chamber with said return passage portion.

5. Apparatus as dened in claim 4 wherein:

said openings are in the form of nozzles, and each of said nozzles isdisposed with its outlet directed towards the stagnant boundary layer onthe trailing surface of the low pressure side of one of said xed guidevanes.

6. Apparatus as defined in claim 4 wherein:

each of said openings is angled inwardly towards the axis of saidcompressor in a direction such that econ-omizer gas discharging fromeach of said openings flows in a direction generally tangent to thecurved trailing surface of one of said guide varies.

7. Apparatus as defined in claim 4 wherein:

said openings are in the form of tapered nozzles capable of increasingthe velocity of economizer gas owing therethrough.

8. Apparatus as dened in claim 4 wherein:

said dividing wall is in the form of a vane plate to which said guidevanes are secured.

References Cited UNITED STATES PATENTS 2,277,647 3/1942 Jones 62-4983,011,322 12/1961 Tanzberger 62--510 MEYER PERLIN, Primary Examiner.

